Control the daemon parameters from the wallet GUI

[cmihai]

Is your feature request related to a problem? Please describe
The Qt wallet allows controlling the following daemon parameters through the Options dialog:
[ ] Size of block storage (-prune command-line option)
[ ] Size of database cache (-dbcache option)
[ ] Number of script verification threads (-par option)
[ ] UPnP port mapping (-upnp)
[ ] Whether to disable automatic outgoing connections (-connect=0)
[ ] SOCKS5 proxy (-proxy)

It also allows overriding the advanced options by editing the configuration file manually. The Electron wallet should also support this feature.

Describe the solution you'd like
The Settings page for the wallet should expose controls for the settings above. After the user changes them and confirms the change, the GUI should restart the daemon with the new options. The changed settings should be persisted in a configuration file in the user's home directory.

The wallet should also allow opening the united configuration file with the user's default editor. After the editor is closed, the daemon should likewise be restarted and the changes applied.